The hexanuclear cobalt-containing tungstovanadate complex, Na17[(Co(OH2)Co2VW9O34)2(VW6O26)]·31H2O (1) has been synthesized through reaction of cobalt nitrate, sodium tungstate, and V2O5 in acetate buffer solution. The title compound has been characterized by X-ray crystallography, elemental analysis, FTIR, UV/Vis, and 51V NMR spectroscopy and by cyclic voltammetry. Complex 1 crystallizes in the monoclinic space group P21/c (a = 15.706(3) Å, b = 21.973(5) Å, c = 35.080(8) Å, α = 90°, β = 97.420(3)°, γ = 90°; V = 12005(5) Å3; Z = 4; based on 192337 independent reflections), which consists of two tri-CoII substituted B-α-[(Co(OH2)Co2VW9O39)] Keggin units bridged by one unique [VW6O16] fragment leading to a banana-shaped structure with idealized C2ν symmetry. Stability studies show that polyoxoanion 1 ultimately transforms into the tetracobalt-containing sandwich-type polyoxoanion, 2, as confirmed by 51V NMR and X-ray crystallographic analyses. Polyoxoanion 1 shows catalytic activity for the H2O2-based epoxidation of 1-hexene and cyclohexene in 1,2-dichloroethane solvent.